Materials which exhibit the phenomenon of total diamagnetism at non-cryogenic temperatures and easily attainable pressures have been the subject of considerable research interest for a significant period of time. That research, which has been referred to as "high temperature superconductor" research, has focused upon the resistive and diamagnetic properties of the materials of interest at 23 degrees Kelvin (K) and above (i.e., 23 degrees above absolute zero). In other words, "high temperature", as that term has been used in the past, referred to temperatures only attainable under extremely cold, namely, cryogenic conditions.
A major advance in the world of superconducting compositions is described in the popular novel, The Breakthrough the Race for the Superconductor written by Robert M. Hazen. The Breakthrough describes the massive scientific effort that was generated when, in 1986, George Bednorz and Alex Muller reported synthesis and testing of a Ba--La--Cu--O composition which exhibited superconductive behavior at 30 degrees K. ("Possible High Tc Superconductivity in the Ba--La--Cu--O System" in Zeitschrift fur Physik, November, 1986). For their efforts, Bednorz and Muller received the Nobel Prize in physics in 1987. Having shown that the perceived 23 degree K. barrier could be broken, the work of Bednorz and Muller spurred the research efforts of many other researchers at several other research laboratories.
Subsequent to the work of Bednorz and Muller, several other researchers synthesized and tested compositions which exhibit superconductive characteristics in the "high temperature" range as that was understood at the time. In other words, those materials exhibited superconductive characteristics at critical temperatures, Tc, of greater than 23.degree. K. As discussed in Breakthrough, Paul Chu (Paul Ching-Wu) and his colleagues synthesized and tested an yttrium, barium, copper system (subsequently known as "1-2-3" reflecting the atomic ratios of the substances) which exhibited Tc of greater than 70 degrees K. This was an important breakthrough because it permitted the creation of superconducting behavior at about the boiling temperature of liquid nitrogen, i.e., 77 degrees K. This achievement (which was characterized as the equivalent of breaking the 4-minute mile in distance racing) permitted research and applications work to proceed using inexpensive, easily handled cryogenic fluid, liquid nitrogen. Research at liquid nitrogen temperatures and above is one of the factors which permitted the phenomenon of superconductivity to emerge from the stage of laboratory curiosity and to begin its Journey to practical application.
In their efforts to understand superconductivity better and thereby, potentially, to increase Tc to even more useful ranges, Chu and others began evaluating superconductor candidates at elevated pressures. At pressures in excess of 10,000 atmospheres (1.times.10.sup.7 k-Pascals), shifts in T.sub.c, either up or down, were regularly observed. As recently as Feb. 10, 1993 Chu has reported (in New Scientist, p. 14) a Tc of 160 degrees K. at a pressure of more than 150,000 atmospheres with respect to a HgBa.sub.2 Ca.sub.2 Cu.sub.3 material. Mercury superconductors were apparently first identified by the CNRS French national research organization in early 1993, c.f. New Scientist, Science, 27 Mar. 1993.
In the first instance, even though considerably more convenient than early superconductor work, cryogenic equipment needed to handle liquid nitrogen at 77K. is expensive, cumbersome, and potentially dangerous. This is particularly true of commercial applications of superconductor materials where cumbersome, expensive, and inconvenient cryogenic equipment is needed to cool the superconductor material to create the desired superconductivity. Clearly, applications which required super atmospheric pressures to reveal the desired elevated Tc would additionally complicate utilization of superconductor compositions as well as increase the expense and inconvenience of doing so.
Also of note is the fact that most materials which have been shown to exhibit superconductivity are oxides. Table 1 entitled "High Temperature Superconductors" at page 12-76 of the 1993-1994 edition of the CRC Handbook of Chemistry and Physics lists approximately 30 materials, only one of which (Rb.sub.2 CsC.sub.80) is not an oxide. (Table 1 indicates preparation in November, 1992.) That non-oxide composition employs carbon, which itself likely has characteristics similar to those exhibited by the metals which comprise the rest of the materials. The highest Tc listed in the above-mentioned Table 1 is 128 degrees Kelvin for a Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10 system.
There is, therefore, a critical need for a material which exhibits superconductivity at non-cryogenic temperatures (i.e., above about 273 degrees K.) and substantially atmospheric pressure. This invention describes such a material.